1. Technical Field of the Invention
This invention relates to telecommunication systems. More particularly, and not by way of limitation, the present invention is directed to a system and method of scheduling radio resources for Quality of Service (QoS) in a wireless communications network.
2. Description of Related Art
Wireless telecommunication networks are evolving from second generation (2G) circuit-switched networks to third generation (3G) packet-switched networks. “Third generation” is the terminology used for packet-switched mobile access systems such as the General Packet Radio Service (GPRS) and the Universal Mobile Telecommunication System (UMTS). The third generation mobile communications system allows multiple mobile users to access and share network radio resources.
Radio resources are limited, and therefore must be managed. There are two objectives of radio resource management: (1) provide the highest attainable Quality of Service (QoS) for individual users, and (2) optimize the utilization of network resources. To achieve these objectives in the UMTS system, new radio resource management algorithms need to be developed for packet admission and scheduling, system load control, and power control.
Packet scheduling is a mechanism that determines which user has the right to transmit in a given time interval. It is well-known that round robin scheduling provides fairness for all users at the expense of user and system throughput. First-In-First-Out (FIFO) scheduling provides optimum user and system throughput for well-behaved traffic if all channels have the same quality. However, wireless channels exhibit frequent variation of quality. To maximize system throughput, radio resources should be assigned to the user with the best channel quality. However, a scheduling methodology based purely on channel quality would be unfair to users with poor channel quality, leading to their potential starvation and excessive delay. Therefore, there is a trade-off between fairness and throughput.
An existing method of scheduling IP packets is described in the IEEE article, “Downlink Radio Resource Management for IP Packet Services in UMTS,” by I. Lopez, P. J. Ameigeiras, J. Wigard, and P. Morgensen (referred to hereinafter as “Lopez”). In Lopez, a modified round robin algorithm is proposed as an intermediate solution for fairness and throughput. At every round robin cycle, a packet from each user is admitted and queued according to arrival sequence. When a user“s packet is scheduled for transmission, the user”s instantaneous channel quality is compared to a threshold historical value. If the channel quality is above the historical threshold, resources are allocated, and the packet is transmitted. Otherwise, the user is moved to the end of the queue. All known schemes use historical and current channel conditions to predict the future channel conditions for packet scheduling.
It is shown in Lopez that the modified round robin methodology provides fairness to all users in the system, and the throughput distribution is very close to that of a normal round robin approach, with some improvement. But the throughput for the modified round robin methodology is significantly less than that of the pure channel quality methodology.
Another solution is described in the IEEE article, “CDMA Forward Link Waterfilling Power Control,” by J. M. Holtzman (referred to hereinafter as “Holtzman”). In Holtzman, a compromise is reached by assigning priorities to all users. A user's priority is defined as the ratio of the user's channel quality to the user's throughput in a given time period. If all users have the same throughput, the user that has the best channel quality is allocated resources. Therefore, the user with the best channel quality will have higher throughput. However, the higher throughput, in turn, reduces the user's priority. Eventually, when priorities are re-calculated at the end of a time period, the process may assign a higher priority to a user with poorer channel quality.
In Holtzman, if the channel quality is the same for all users, or if the time period “window” used for calculating past throughput is too short, throughput will approach the round robin methodology. If the window is too long, this method will provide slow response for user priority and will lead to unfairness to users with poor channel quality.
In order to overcome the disadvantage of existing solutions, it would be advantageous to have a system and method of scheduling radio resources that achieves the throughput level of the pure channel quality methodology while also achieving fairness close to the round robin methodology. The present invention provides such a system and method.